1. Field of Invention
This invention relates to the use of the Vibrio cholerae accessory cholera enterotoxin (Ace) and Ace analogs as novel activators of the calcium-dependent chloride channel. More particularly, this invention relates to the use of Ace and Ace analogs to treat diseases involving insufficient chloride transport or insufficient bicarbonate transport, such as cystic fibrosis.
2. Introduction to Cystic Fibrosis
Cystic fibrosis (CF) is the most common genetic disease in Caucasian populations, with an incidence of 1 in 2,000 live births and a carrier frequency of approximately 1 in 20. It is inherited as an autosomal recessive disease. The cystic fibrosis gene has been mapped, cloned, and sequenced (Rommens et al., Science 245:1059-1065 (1989); Kerem et al., Science 245:1073-1080 (1989)). The gene product is the cystic fibrosis transmembrane regulator (CFTR) which functions as a chloride ion channel in the apical membranes of secretory epithelial cells (Anderson et al., Science 251:679-682 (1991)). The expression of the CFTR is most prominent in sweat glands and the respiratory and gastrointestinal tracts (Collins, F. S., Science 256:774-779 (1992)). CF is a disease of the epithelial cells, and the distribution of CFTR is essentially consistent with the clinical pathology.
In CF, the functionally defective apical membrane chloride channel secondarily leads to a loss of luminal sodium and water. In airways, the increase in sodium absorption and reduction in chloride secretion both lead to a loss of airway surface water. Thus, airway mucus is inspissated because of insufficient endobronchial water. Bronchiolar plugging and decreased mucociliary clearance follow. These changes in the pulmonary environment result in an increase in bacterial colonization. The colonization of the lung leads to a cycle of inflammation, destruction, and further colonization. The microorganisms colonizing the lungs attract neutrophils which contain and resolve a pulmonary infection in the normal host. However, in the patient with CF, the release of proteolytic enzymes by neutrophils further damages lung parenchyma. Neutrophils release neutrophil elastase which causes many typical pathologic features of CF, including epithelial damage, bronchial gland hyperplasia (leading to increased mucus production), and connective tissue damage resulting in bronchial distortion (Stockley et al., Clin. Sci. 74:645-650 (1988)). Neutrophils also release serine protease, a factor that damages bronchial cilia (Cole, P. J., Eur. J. Respir. Dis. 69(suppl 147):6-15 (1986)). The damaged bronchial cilia have decreased ciliary beat frequency and a reduction in mucociliary clearance. In the normal host, neutrophil proteases are controlled by the naturally occurring protease inhibitors found in the pulmonary tree. The most potent of these, alpha-1-antitrypsin, irreversibly binds with high affinity to serine protease. However, in the lung of the patient with CF, proteases are produced by a variety of cells besides neutrophils, including pulmonary macrophages and the microorganism, Pseudomonas aeruginosa (Fick et al., Chest 95:215S-216S (1989)). This excessive protease production overwhelms the naturally occurring endogenous protease inhibitors.
As a result of these physiological changes, respiratory tract diseases are responsible for more than 90% of the morbidity and mortality in CF (Hata et al., Clin. Chest Med. 9:679-689 (1988)).
The mainstays of treatment of cystic fibrosis are antibiotics for clinical exacerbations of pulmonary infection, aggressive physiotherapy and bronchodilators to increase the rate of secretion removal, and proper nutrition. (Hata et al., (1988)). There are also a number of new modalities which are under investigation. These include gene therapy (Drumm et al., Cell 62:1227-1233 (1990)), corticosteroids (Rosenstein et al., Pediatrics 87:245-246 (1991)), nonsteroidal anti-inflammatory agents (Konstan et al., Am. Rev. Respir. Dis. 141:186-192 (1990)), treatment with alpha-1-antitrypsin (McElvaney et al., Lancet 337:392-394 (1991)), and DNase (Elms et al., Thorax 8:295-300 (1953)). However, none of these treatment modalities are effective for long-term survival of the CF patient.